1. Field of the Invention
This invention relates to an image-forming lens such as an objective or collimator lens which is used in a video disk, audio disk or optical memory apparatus, and more particularly to a distributed index image-forming lens for an optical memory which has a numerical aperture NA of not more than about 0.5 and the aberration of which in the vicinity of the optical axis is preferably corrected.
2. Description of the Prior Art
As heretofore known, the distributions of the refractive indexes of distributed index lenses are classified into three fundamental refractive index distributions: radial, axial and spherical types. The radial type is defined by a refractive index distribution which is aligned in the radial direction perpendicular to the optical axis; the axial type is defined by a refractive index distribution which is aligned in the direction of the optical axis; and the spherical type has a refractive index distribution which exhibits a spherical symmetry about a point. Many proposals for using these distributed index lenses as objective lenses for optical disks have been made, such as in Japanese Patent Laid-Open No. 122512/1983, No. 62815/1984, No. 140309/1985 and No. 172010/1985. All of these proposals relate to single lenses having refractive index distributions of the radial or axial type alone. On the other hand, an image-forming system of the spherical type is disclosed in Japanese Patent Laid-Open No. 181516/1982. This system, however, employs a specific arrangement in which a core having a spherical refractive index distribution is coated with a cladding of a uniform medium. Such an arrangement is not preferable since it is difficult to manufacture, and it cannot facilitate the correction of any aberration of the spherical lens so long as the lens has a spherical refractive index distribution and a numerical aperture NA of not more than about 0.5.